The present invention relates to a reflection mirror in an image reading device for a scanner apparatus, a copier, a facsimile machine, or the like which optically reads document images, and in particular, to improvements in a reflection mirror unit for an image reading device in which an optical system such as a light source or a mirror reciprocates under a document on a platen for scanning.
In general, in image reading devices for scanner apparatuses, copiers, and facsimile machines, a document placed on a plate is irradiated with light from a light source to form reflected light from the document into an image, which is then electrically read. A line sensor is used as a photoelectric conversion element, and a line light source is used as a light source. The light source is allowed to perform a scanning operation in a sub-scanning direction along the document to read the planar image.
In order for an image forming optical system to read such a document image in order of lines, it is necessary to move, in a sub-scanning direction, the light source and a refection mirror guiding reflected light from the document to the photoelectric conversion element. The light source and the reflection mirror are mounted on a carriage that can reciprocate along the document surface so that the movement of the carriage allows the document surface to be scanned. In this case, in a known device, the light source and the reflection mirror are mounted on a single carriage to constitute an image forming optical system. Another device has a first carriage and a second carriage. The light source is mounted one of the first and second carriages, while the reflection mirror is mounted on the other carriage. The first and second carriages are moved to constitute an image forming optical system.
In any of the above device configurations, the device is generally configured to have a thin, flat shape, and further designed to irradiate a document on the platen located at the top of the device with light from the light source and use the reflection mirror to reflect reflected light so as to form an optical path in a longitudinal direction (horizontal direction) of the device. For example, Japanese Patent Publication No. 1-116662 (TOKKAI) discloses a device having a first carriage equipped with a light source lamp and a first reflection mirror that polarizes reflected light from a document, and a second carriage equipped with a second reflection mirror and a third reflection mirror which polarize light from the first reflection mirror. In this scanning device, the first and second carriages are supported so as to be moved along a platen. A driving motor moves the first carriage at a predetermined speed and moves the second carriage at a speed half of the predetermined speed of the first carriage. A condensing lens forms an image on a photoelectric element located on a device substrate.
Similarly, Japanese Patent Publication No. 2004-77851 (TOKKAI) discloses a device corresponding to the above configuration which additionally has a carrier frame slidably supported by a guide rail and a mirror supporting frame that supports the pair of reflection mirrors mounted on the second carriage, and the carrier frame and mirror supporting frame are composed of individual sheet metal members and integrated together by screws.
When the paired reflection mirrors are integrated into a unit that is movable along the platen as described above, the mirror supporting frame is conventionally mounted on a base frame carrying the unit and slidably supported by the guide rail, as disclosed in Japanese Patent Publication No. 2004-77851 (TOKKAI). The two separate frames are used to support the pair of reflection mirrors because dimensional precision can be prevented from varying, and it facilitates positional adjustments during assembly of a metal material in order to rigidly construct the unit and determine a position, in this case, by press-working, where the mirrors are supported.
For example, as shown in FIG. 11(a), if a pair of reflection mirrors 50a, 50b is prevented from being mounted on a mirror support frame 51 at a predetermined angle in the correct posture, the optical path may be bent. If the mirror support frame 51 is not accurately mounted on a base frame 52 at the correct position during the scanning movement, the mirrors may be vibrated or the optical path direction may be shifted. Thus, the mirror support frame 51 comprises a stay like frame 51a having bent pieces 51b, 51c at the opposite ends thereof which are bent by press-working and to which the respective ends of each reflection mirror are mounted by positioning the ends at corresponding surface references. The mirror support frame 51 is fixed to the base frame 52 with screws.
When the reflection mirror unit is constructed by connecting the two frame members together as described above, a machining operation is easy. Further, since the bent pieces, to which the refection mirrors are mounted, are integrally formed by press-working, for example, the positions of a right mirror attachment portion and a left mirror attachment portion need not be adjusted, and this configuration results in easy assembly. If this assembly method is employed to the conventional technique, since the mirror support frame is fixed to the base frame with screws or the like as described in Patent Document 2, the mirror support frame may be mounted so as to incline in the direction of arrow a in the figure (see FIG. 11(a)).
The bent pieces, which simultaneously support the mirrors, are likely to be bent in the direction of arrow b (see FIG. 11(b)). Consequently, when the right and left bent pieces are bent during assembly or the like, the reflection mirrors may disadvantageously mounted at a slant. When the two frames are bent at the mounting positions during a connection operation, the reflection mirrors are prevented from being placed at the accurate positions in the correct postures. This may bend the optical path or change the radius of curvature, disadvantageously distorting a read image. Thus, tilting the pair of reflection mirrors in the direction of arrow a may blur the document image or distort the read image. Further, misalignment in the direction of arrow b leads to misalignment in the sub-scanning direction shown by Δy, resulting in a blurred or distorted image. This may in turn result in misalignment in the main scanning direction shown by Δx in the figure, causing an image reading start position to deviate from the correct position. In particular, it is difficult to form the surface of each mirror perfectly flat, and the surface of the mirror normally tends to have an insignificant aberration in a central region, but it tends to have a significant aberration in a peripheral portion. Thus, a very difficult operation needs to be performed in order to assemble the mirrors while preventing the misalignment of the center position of the mirror during assembly.
It is thus a main object of the present invention to provide a reflection mirror unit that, when a mirror support frame carrying a pair of reflection mirrors is connected to a base frame supported by a guide rail, enables the reflection mirrors to be held at accurate positions in correct postures in the main scanning direction and in the sub-scanning direction.
It is another object of the present invention to provide a reflection mirror unit in which a mirror support frame carrying a pair of reflection mirrors is composed of a relatively thin plate of metal or the like, which enables the reflection mirrors to be reliably held, as well as an image reading device using the reflection mirror unit.
Further objects and advantages of the invention will be apparent from the following description of the invention.